1. Field of the Invention
This invention relates to coated steel sheets each provided with an electrodeposition painting having a superior appearance, and more particularly, relates to a coated steel sheet provided with an electrodeposition painting having a superior appearance, in which the coated steel sheet is preferably used for automobile bodies, home electrical appliances, and the like and is capable of forming a superior finished coat on a surface of the steel sheet which is rubbed in press forming and the like.
2. Description of the Related Art
A coated steel sheet comprising a zinc-based plated layer has superior corrosion resistance and has been widely used in, for example, automobile bodies and home electrical appliances. In particular, in Japan, a coated steel sheet comprising a zinc-nickel alloy plated layer or a zinc-iron alloy plated layer has been primarily used for automobile applications.
A coated steel sheet comprising a zinc-nickel alloy plated layer is manufactured by an electroplating method, and in this method, the content of nickel (Ni) is strictly controlled in a very narrow region (generally, 12 ±1 mass %). Hence, when the coated steel sheet described above is manufactured, a highly advanced manufacturing technique must be used, and in addition, due to the use of Ni, the manufacturing cost is increased. On the other hand, a coated steel sheet comprising a zinc-iron alloy plated layer is manufactured by a hot-dip plating method or an electroplating method. When the coated steel sheet described above is manufactured, since the content of iron must be precisely controlled, a highly advanced manufacturing technique must be used. In addition, when a hot-dip plating method is used, since the plated layer obtained thereby is inferior in terms of press formability as compared to the zinc-nickel alloy plated layer described above, a second coating layer is generally formed on the surface of the plated layer for improving the press formability.
In recent years, in the automobile manufacturing industry, concomitant with the trend toward the globalization and sharing of automobile components, the use of a coated steel sheet, which is inexpensive and which can be easily manufactured, has been increased all over the world, and in Japan, the use of a zinc plated steel sheet (manufactured by electroplating or hot-dip plating, not followed by heating for alloying) has begun to take over the use of a coated steel sheet comprising a zinc-iron alloy layer manufactured by hot-dip plating and a coated steel sheet comprising a zinc-nickel alloy plated layer manufactured by electroplating. In Europe, performances of press dies, welders, and the like are improved by users of coated steel sheets, and in order to improve the press formability thereof, for example, a zinc phosphate layer is formed on a plated layer by producers of coated steel sheets. The zinc phosphate layer thus formed as a second coating layer has the effect of retaining oil in gaps between crystal grains thereof and also has the effect of preventing the zinc plated layer from being directly brought into contact with a press die, that is, the effect of functioning as a buffer, and hence it has been believed that the press formability is improved.
Among the coated steel sheets, for example, a zinc plated steel sheet used for outer plates of automobile bodies or the like is frequently processed by press forming in an automobile manufacturing process. Subsequently, the zinc plated steel sheet thus pressed is processed by electrodeposition painting to form an under painting thereon to rustproof the steel sheet, followed by finish painting performed using a spray system, such as automobile primary painting and top painting, for a good external appearance, thereby forming an automobile component.
The external surfaces of automobiles are particularly important in appearance, and hence superior clarity after painting has been required. As a coated steel sheet which can meet the requirement described above, a coated steel sheet provided with an electrodeposition painting having small surface irregularities is preferably used.
When a zinc plated steel sheet processed by phosphate treatment is used, the appearance of an electrodeposition painting formed on a surface, which is rubbed in press forming, of the steel sheet described above is inferior in some cases, and a panel formed from the steel sheet described above must be disadvantageously processed by surface treatment, such as polishing so that the surfaces are smoothed, in an automobile manufacturing process.
In a coated steel sheet, irregularities of a surface of the base steel sheet are naturally reflected in those of the surface obtained after an electrodeposition painting is performed. In addition, when the surface resistance of the coated steel sheet is large, an electrodeposition painting having an irregular thickness is formed since current flows unevenly through local areas during electrodeposition painting, and as a result, a base steel sheet having small surface irregularities may have large surface irregularities after electrodeposition painting in some cases.
In the related coated steel sheet described above, for example, when a film having a large surface resistance, such as an organic film, is provided on the surface of the base steel sheet, a method for decreasing the surface resistance of the steel sheet by imparting hydrophilic properties has been proposed (Japanese Patent No. 2922426 and the like).
However, the method or the technique described above for obtaining a superior painting appearance, in which the surface resistance of the base steel sheet is decreased to improve the uniformity of an electrodeposition painting itself, can only be applied to an organic composite coated steel sheet which comprises an organic film on the surface the base steel sheet.
In the case other than the organic composite coated steel sheet, only one method has been proposed for improving the coating appearance, in which the surface roughness of a base steel sheet itself is decreased.
For example, Japanese Unexamined Patent Application Publication No. 9-263967 discloses a method in which a superior painting appearance is obtained by controlling the roughness of a base steel sheet. In that method, the product of a filtered center-line waviness (Wca) and a peak per inch (PPI) of a surface of a hot-dip zinc plated steel sheet is controlled to be 40 or less, or Wca and PPI are controlled to be 0.5 μm or less and 80 or less, respectively, to decrease surface irregularities of the base steel sheet itself for improving the clarity after the painting. However, according to the method described above, the painting is performed by paint application or roll coating. Hence, electrodeposition painting is not used. Since the irregularities of the surface of the base steel sheet are reflected, the clarity after painting is improved when the Wca and PPI are decreased. However, the appearance of an electrodeposition painting, which is influenced by the surface resistance and is formed on a surface of the steel sheet to be rubbed in press forming, is not described at all.
In addition, Japanese Examined Patent Application Publication No. 5-83628 has disclosed an alloyed hot-dip zinc-plated steel sheet (galvannealed steel sheet) in which the surface roughness of a base steel sheet is controlled. In that publication, the roughness Ra and PPI of the base steel sheet are set to 1.0 μm or less and 250 or more, respectively, to improve the press formability by decreasing frictional resistance to sliding of a die. The reason the PPI is set to 250 or more relates to a specific alloy crystal structure obtained by alloyed hot-dip zinc plating, there is no description about the appearance of an electrodeposition painting formed on a surface of the steel sheet which is rubbed in press forming, sliding, or the like.
In addition, Japanese Unexamined Patent Application Publication Nos. 6-246306 and 6-269803 disclose a steel sheet having superior painting clarity and press formability. In the steel sheet mentioned above, the Ra at a concave portion of the steel sheet is set to 0.8 μm or less, and the size of a peak of a convex portion and the distance between the convex portions are controlled. In addition, the height of a convex portion of the steel sheet at an inner surface side in press forming is set to be larger than that at an outer surface side, the area ratio of the concave portions at the inner surface side is set to 70% to 96%, and the average area ratio thereof is set to be smaller than that at the outer surface side, thereby obtaining superior coating clarity even after press forming. That is, the roughness profiles of the front and the rear surfaces are made different from each other so that the superior painting clarity is obtained even after press forming. Among the factors relating to the roughness, the Ra of the concave portions, the area ratio thereof, and the like are determined to obtain superior clarity after painting, and the specific values of the convex portion are determined to improve the press formability. In addition, the reason the painting clarity after press forming is degraded is that the surface roughness of a steel sheet is changed by press forming and an undulation component on one side surface of the steel sheet is clearly transferred on the opposite surface thereof by a pressure applied thereto. Hence, the roughness profiles on the front and the rear surfaces are made different from each other. In that method, in consideration of the roughness after press forming, the change of the surface roughness caused by press forming is estimated beforehand, and the roughness profiles on the front and the rear surfaces are determined to decrease the change thereof. Hence, it is considered that the method described above may be effectively applied to a cold-rolled steel sheet or a coated steel sheet containing only one layer. However, sufficient improvement cannot be obtained by the method described above for a coated steel sheet containing at least two layers. In addition, in the method described above, for example, since the roughness control must be differently performed between the front and the rear surfaces, the area, the height, and the like of the convex portions must be controlled, and a roller must be frequently replaced, the manufacturing of products may become complicated in some cases.
In addition, in the related techniques described above, the evaluation is performed by using a cold-rolled steel sheet or a coated steel sheet right after the production thereof. That is, in Japanese Unexamined Patent Application Publication Nos. 6-246306 and 6-269803, concerning the paintability, there has been no description about the appearance of an electrodeposition painting formed on a surface of the steel sheet which is rubbed in press forming, sliding, or the like.
However, in an actual automobile manufacturing process, after processing by press forming, welding, polishing performed together with an appearance check, and like, a coated steel sheet is conveyed to a painting step, and after treatment using phosphate is performed as a pre-treatment step, electrodeposition painting is performed. Accordingly, although a steel sheet having superior painting clarity can be obtained from a coated steel sheet by a laboratory experiment based on the related techniques described above, when the steel sheet described above is treated by electrodeposition painting after being processed by the manufacturing steps described above, the appearance of the electrodeposition painting may be degraded in some cases. This appearance is apparently different from the appearance of the electrodeposition painting formed on the surface of the coated steel sheet, which is not rubbed in press forming, sliding, or the like, by the laboratory experiment described above. From the result described above, the evaluation of the appearance of an electrodeposition painting formed on a surface of a coated steel sheet, the surface being rubbed in press forming beforehand as is that used for automobile applications or the like, is apparently different from the evaluation of the appearance of the electrodeposition painting formed on a surface of a coated steel sheet by a laboratory experiment, the surface not being rubbed in press forming, sliding, or the like. That is, it is clearly understood that, even when a superior evaluation result of the appearance of the electrodeposition painting is obtained by a laboratory experiment, since the use conditions of the coated steel sheet are apparently different from those at the user side, the evaluation result obtained by a laboratory experiment naturally different from that obtained at the user side. Hence, to obtain the same evaluation result as that obtained under actual use conditions, the appearance of an electrodeposition painting formed on a surface of a steel sheet, which is rubbed in press forming or is processed by a friction test before electrodeposition painting, is evaluated in a laboratory experiment. In addition, at the user side where a coated steel sheet is used, to improve the appearance having surface damages generated in press forming, polishing of surfaces of the coated steel sheet may be performed before electrodeposition painting in some cases, and as a result, depending on type of coated steel sheet, a problem may arise in some cases in that a polishing pattern is clearly observed on the surface of the steel sheet after electrodeposition painting is performed. In the case described above, it is also understood that when electrodeposition painting is performed on a surface of a steel sheet, which is actually polished, the appearance of the electrodeposition painting can be correctly evaluated.